Mechanics

SERVICE 4WD MESSAGE

Chevrolet Silverado

I keep getting a message to service the 4WD on the message center.
How can I determine what is causing this?
Will a DTC reader/scanner tell me the error code?
2000 Silverado with 32000 miles on 4.8L


Thanks, : )
Avatar
Cruisey
November 16, 2007.



Yes, you should have it scanned for codes. Then post the codes for us please.

Dennyp
Nov 17, 2007.
Thanks dennyp, I ordered a EQUUS 3100 - CanOBD2 Diagnostic Tool, as soon as I receive it I'll let you know what DTC's come up.

Tiny
Cruisey
Nov 18, 2007.
Well, I hooked up the scanner and it said there are no DTC's stored in the vehicles memory.
I guess I will try to reset the monitors and do a OBDII drive cycle with the scanner hooked up until the Service 4WD warning light comes on again.

Tiny
Cruisey
Nov 27, 2007.
Today I pushed the erase button on the scanner to reset the monitors.
After driving about 5 miles with the scanner hooked up the Service 4WD warning came on.
Still no DTC's
I'm thinking maybe a bad relay? (The relay that turns on the Service 4WD warning)
I'm buying a Toyota next time!

Tiny
Cruisey
Nov 28, 2007.
Your scanner might not be able to read transfer case DTCs. I use a TECH2 scanner. They run around $4800.00

Dennyp
Nov 28, 2007.
OK, thanks denny
I guess I will take it to the dealer because I'm sure not going to buy one of the scanners you have.
I'll let you know what they find.

Tiny
Cruisey
Nov 28, 2007.
The DTC that came up was B2725
B2725 - ATC Mode Switch Circuit Malfunction

Here's a list of B codes.

http://www.2carpros.com/trouble_codes/obd2

I don't understand why GM doesn't have the code display on the message center instead of the Service 4WD message.
But then, look how long it took them to make it so you couldn't leave your headlights on. :shock:

Tiny
Cruisey
Dec 4, 2007.
I have had the same problem with my 1500HD with All wheel drive tranny.

It's out of warranty but only has 28k miles and this message has been a problem. I had the switch module changes 2x. Now, since out of warranty, I no longer take it to the Chevy dealer because of bad service. I brought to my regular guy but his testing unit couldn't come up with a code. He said to take it to Chevy because of a Reflash bulletin he found.

I'm hoping the reflash works because the alternative is quite costly. $500 for the motor module and $200 for the front axle module.

Let me know how you make out and maybe we can compare notes. : D

Tiny
Jgentile
Dec 8, 2007.
Here is a copy of a service bulletin. If your symptoms match this, follow the directions.

Subject: Service 4WD Indicator Illuminated, DTC B2725 Set (Replace Transfer Case Shift Control Switch) #02-04-21-004 - (04/15/2002)

Models: 1999-2002 Chevrolet and GMC C/K Pickup and Utility Models (Silverado, Sierra, Avalanche, Suburban, Tahoe, Yukon, Yukon XL)

with Four Wheel Drive and NVG 236/246 Transfer Case (RPO NP8)

Condition
Some customers may comment on the Service 4WD indicator illuminating. Upon investigation, the technician may find DTC B2725 set.

Cause
The condition may be due to a intermittent fault in the transfer case shift control switch.

Correction
Important: The transfer case shift control module should not need replacement. DO NOT replace P/N 15749703 unless diagnosis points to the module as the malfunction.

Replace the transfer case shift control switch using published Service Information procedures.

Parts Information
Part Number
Description

15709327
Transfer Case Shift Control Switch

Parts are currently available from GMSPO.

Dennyp
Dec 9, 2007.
Here is a diagnostic chart that might also be of some help to you.

Document ID# 525340
2000 Chevrolet Chevy K Silverado - 4WD

DTC B2725 ATC Mode Switch Circuit Malfunction

Circuit Description
The range/mode switch circuit consists of four normally open switches. The transfer case shift control module supplies a regulated 8 volt, DC to the switch through the 8 V Reg circuit. The current travel through a 9.09 K ohms resistor located inside the switch. It returns current to the transfer case shift control module through the switch signal circuit.

The transfer case shift control module constantly monitors this signal voltage to determine the condition of the mode switch circuit. If no buttons are pressed, and the transfer case shift control module detects a voltage level outside the possible range (approx.-0.5 to 1.0 volts) for longer than 5 minutes, the transfer case shift control module will set this DTC or if a button is held down or sticks for a period longer than 5 minutes.

When each of the switches is depressed they will complete a circuit through their own specific resistor. The transfer case shift control module continuously monitors the switch input to determine whether the 4HI, AUTO 4WD, 2HI, and 4LO button selections are made by the driver. Neutral may be obtained if the following conditions are met: The engine is running.
The automatic transmission is in neutral (or the clutch pedal is depressed on a manual transmission application).
The vehicle speed is below 3 MPH.
The transfer case is in the 2HI mode.
Once these conditions have been met, pressing and holding both the 2HI and 4LO buttons for 10 seconds will shift the transfer case into neutral, turning on the red neutral indicator lamp.

Conditions for Setting the DTC
The system constantly monitors the voltage on the switch signal circuit.
If the system detects a voltage level outside the possible range produced when no buttons are depressed, (approx.-0.5 to 1.0 volts), the DTC is logged.
Action Taken When the DTC Sets
All shifting will be disabled.
The SERVICE indicator (4WD/AWD) lamp will be latched on for the remainder of the current ignition cycle.
Conditions for Clearing the DTC
The transfer case shift control module will clear the current DTC if the condition for setting the DTC no longer exists.
A history DTC will clear after 100 consecutive ignition cycles without a fault present.
History DTCs can be cleared using a scan tool.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.

Tests for proper operation of the transfer case mode select switch.

Tests for proper voltage supply to the transfer case mode select switch.

Tests the transfer case mode select switch for proper resistance values in all switch states.

Tests the switch signal circuit for a short to voltage.

Tests the switch signal circuit for an open or short to ground.

Tests the switch signal circuit for a short to the 8 V Reg circuit.

Determines whether the 8 V Reg circuit is greater than or less than the specified voltage.

Tests the 8 V Reg circuit for a short to voltage.

Tests the 8 V Reg circuit for an open or short to ground.

DTC B2725 Mode Switch Circuit Malfunction Step
Action
Value(s)
Yes
No

1
Was the Transfer Case Diagnostic System Check performed?
--
Go to Step 2
Go to Diagnostic System Check

2

Important: Neutral Range will only be obtained by pressing and holding both the 2 HI and the 4 LO buttons simultaneously for 10 seconds. Refer to Circuit Description.

Install a Scan Tool.
Start the engine.
Place the transmission into NEUTRAL (or clutch pedal depressed on the manual transmission applications).
Set the park brake.
With a scan tool, observe the Commanded Mode Indicator data parameter in the Transfer Case data list.
Depress each of the Mode/Range buttons.
Does the scan tool indicate that the Commanded Mode Indicator data parameter is indicating the correct state?
--
Go to Testing for Intermittent and Poor Connections
Go to Step 3

3
Turn the ignition to OFF.
Disconnect the selector mode switch.
Turn the ignition to ON, with the engine OFF.
Measure the voltage from 8 V Reg Circuit to a good ground.
Does the voltage measure within the specified range?
7-9 V
Go to Step 4
Go to Step 8

4
Connect a DMM between the Selector Mode Switch Signal Circuit and the 8 V Reg circuit at the switch.
Measure the resistance through the selector mode switch while depressing the range buttons and comparing the values.
Does the DMM indicate all resistance values within the specified ranges?
Normal 8.63-9.54 K ohms Auto 4 WD 61.7-68.1 ohms 4 HI 656-670 ohms 2 HI 1.50-1.53 K ohms 4 LO 2.32-2.37 K ohmsNeutral 1.015-1.035 Kohms
Go to Step 5
Go to Step 11

5
Turn the ignition OFF.
Disconnect the transfer case shift control module.
Turn the ignition ON, with the engine OFF.
Test the switch signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Was the condition found and corrected?
--
Go to Step 13
Go to Step 6

6
Test the switch signal circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Was the condition found and corrected?
--
Go to Step 13
Go to Step 7

7
Test the switch signal circuit for a short to the 8 V Reg circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Was the condition found and corrected?
--
Go to Step 13
Go to Step 12

8
Was the voltage measured in Step 3 greater than the specified value?
9 V
Go to Step 9
Go to Step 10

9
Turn OFF the ignition.
Disconnect the transfer case shift control module.
Turn ON the ignition, with the engine OFF.
Test the 8 V Reg circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Was the condition found and corrected?
--
Go to Step 13
Go to Step 12

10
Turn OFF the ignition.
Disconnect the transfer case shift control module.
Turn ON the ignition, with the engine OFF.
Test the 8 V Reg circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Was the condition found and corrected?
--
Go to Step 13
Go to Step 12

11
Replace the selector mode switch. Refer to Transfer Case Shift Control Switch Replacement.

Is the repair complete?
--
Go to Step 13
--

12
Replace the transfer case shift control module. Refer to Transfer Case Shift Control Module Replacement.

Is the repair complete?
--
Go to Step 13
--

13
Use the scan tool in order to clear the DTCs.
Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.
Does the DTC reset?
--
Go to Step 2
System OK

Document ID# 525340
2000 Chevrolet Chevy K Silverado - 4WD

Dennyp
Dec 9, 2007.
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